1. Field of the Invention
The present invention relates to an optical component applicable to the field of optical communications or the like and, in particular, to an optical filter comprising a long-period grating for eliminating the wavelength dependence of the gain of a fiber amplifier doped with rare earth.
2. Related Background Art
A typical optical fiber communication system comprises an optical transmitter including a light source, an optical fiber line having one end connected to the optical transmitter, and an optical receiver connected to the other end of the optical fiber line. An optical amplifier for amplifying signal light in a predetermined wavelength band is installed in the optical fiber line. Such an optical fiber communication system often utilizes WDM signals in the band of 1.5 xcexcm and employs, as its amplifier, a fiber amplifier doped with rare earth such as Erbium (Er) or the like. This erbium-doped fiber amplifier (EDFA) forms a population inversion of electron state within the EDFA in response to excitation light having a predetermined wavelength, and induces stimulated emission in response to light in the band of 1.5 xcexcm incident thereon, thereby amplifying the incident light.
In such an optical fiber communication system, the amplified spontaneous emission (ASE) generated by the mutual action between the power of excitation light and Er ions within the EDFA becomes a noise component. The ASE lowers the gain and increases the noise figure. Also, since the ASE has a power distribution peaking at 1.53 xcexcm, when optical amplification is repeated by a plurality of EDFAs, their gain may fluctuate among individual wavelength components of signal light (a wavelength dependence may occur in the amplification gain of the optical amplifier) under the influence of the power distribution of ASE. As a consequence, in a WDM (Wavelength Division Multiplexing) communication system for transmitting a plurality of signal light components having wavelengths different from each other, different gains are given to the respective channels (respective signal light components), whereby the bit error rate may become higher in some channels.
A technique using a long-period grating for overcoming these problems is disclosed in a paper titled xe2x80x9cBroad-Band Erbium-Doped Fiber Amplifier Flattened Beyond 40 nm Using Long-Period Grating Filterxe2x80x9d (IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 10,xc2x0OCT. 1997, pp. 1343-1345).
This long-period grating is a portion of the core region where the refractive index periodically changes along the axis of an optical waveguide, and is a grating which induces coupling between a core mode and a cladding mode in the signal light propagating through the optical waveguide. The period (pitch) of the grating is set such that the optical path difference between the core mode and cladding mode within one period becomes identical to a predetermined wavelength, and thereby yielding a strong power conversion from the core mode to the cladding mode. As a result, since the long-period grating acts to radiate the core mode to the cladding mode, the intensity of the core mode over a narrow band centered at a predetermined wavelength (loss wavelength) is attenuated.
The center wavelength of the wavelength spectrum of light radiated from the core to the cladding by the long-period grating, i.e., loss wavelength, is determined according to the following expression (1):
xcex2core(lm)xe2x88x92xcex2clad(n)=2xcfx80/xcex9xe2x80x83xe2x80x83(1) 
where l and m are the order of the core mode (1=0, m=1 in the fundamental mode LP01), xcex2core (lm) is the propagation constant defined by (lm), xcex2clad(n) is the propagation constant of the n-th order cladding mode, and xcex9 is the grating period of the long-period grating.
Since the propagation constants xcex2core(lm), xcex2clad(n) are parameters dependent on the wavelength, the loss wavelength of the long-period grating can be controlled when the long-period grating is formed with its grating period xcex9 being adjusted in view of the above-mentioned expression (1).
On the other hand, xcex2core(lm), xcex2clad(n) are dependent on the effective refractive indexes of the core and cladding, respectively. Consequently, when the grating period xcex9 is set constant, the loss wavelength of the long-period grating mainly depends on the difference between the effective refractive index of the core and the effective refractive index of the cladding in the part formed with the long-period grating (grating forming region). Also, the effective refractive index of the core in the grating forming region can be considered in terms of the average value of modulated refractive index. Consequently, the difference between the effective refractive index of the core and the effective refractive index of the cladding in the grating forming region depends on the average refractive index of the core and the average refractive index of the cladding. Further, the amount of refractive index change (amplitude of refractive index modulation) in the region doped with GeO2 changes in response to the irradiation amount of ultraviolet light for forming the grating. That is, the refractive index of the core itself also changes in response thereto. Eventually, the loss wavelength of the long-period grating can be controlled also by forming the long-period grating with the irradiation amount of ultraviolet light being adjusted, so as to regulate the difference between the effective refractive index of the core and the effective refractive index of the cladding.
As a result of studies of conventional optical filters, the inventors have found the following problems. Namely, since the gain of an EDFA such as that mentioned above depends on the wavelength, the optical power may fluctuate among the individual wavelength components of the WDM signal light amplified by the EDFA. Therefore, for wavelength components with higher and lower gains, long-period gratings exhibiting greater and smaller amounts of loss are prepared (and inserted in the transmission line), respectively, so as to homogenize the gain.
In general, however, in an optical filter formed by a combination of a plurality of long-period gratings, the overall transmission characteristic of the optical filter would not become the product (or sum in terms of dB) of the transmission characteristics of individual long-period gratings, thereby making it difficult to yield an optical filter having a desirable transmission characteristic as a whole.
In order to overcome the above-mentioned problems, it is an object of the present invention to provide an optical filter which, when applied to an optical transmission system having an optical amplifier, can eliminate the wavelength dependence of gain in the optical amplifier, while having a structure which can be made easily; and a method of making the same.
The optical filter according to the present invention comprises an optical waveguide having a core region with a predetermined refractive index and a cladding region, provided on an outer periphery of the core region, with a refractive index lower than that of the core region, wherein a plurality of long-period gratings are arranged, at least, in the core region. Here, as explicitly indicated in U.S. Pat. No. 5,703,978 as well, the above-mentioned long-period grating is a grating which induces coupling (mode coupling) between core mode light and cladding mode light which propagate through an optical waveguide such as optical fiber, and is clearly distinguishable from a short-period grating which reflects a light component having a predetermined wavelength. Also, for yielding a strong power conversion from the core mode to the cladding mode, the grating period (pitch) in the long-period grating is set such that the optical phase difference between the core mode light and the cladding mode light becomes 2xcfx80. Thus, since the long-period grating acts to couple the core mode to the cladding mode, the core mode attenuates over a narrow band centered at a predetermined wavelength (hereinafter referred to as loss wavelength).
Among the plurality of long-period gratings, a first long-period grating attains an attenuation peak at a first wavelength by mode coupling, whereas a first filter region provided with the first long-period grating has a refractive index fluctuating with a first period. Further, among the plurality of long-period gratings, a second long-period grating attains an attenuation peak at a second wavelength, different from the first wavelength, by mode coupling, whereas a second filter region provided with the second long-period grating has a refractive index fluctuating with a second period.
In particular, in the optical filter according to the present invention, the order of cladding mode in the light attenuated by the first long-period grating to couple with its core mode differs from the order of cladding mode in the light attenuated by the second long-period grating to couple with its core mode.
In this configuration, in the respective light components having the first and second wavelengths propagating through the optical waveguide, their core modes are coupled to different orders of cladding modes by their corresponding first and second long-period gratings. Then, attenuation peaks centered at the respective wavelengths are generated. Here, the first attenuation peak is not influenced by other cladding modes having different orders. Namely, since the independence of transmission characteristic can be secured for each long-period grating in the optical filter according to the present invention, a plurality of predesigned long-period gratings may be combined together, thus making it easy to design an optical filter having a desirable transmission characteristic.
In the optical filter according to the present invention, the first and second filter regions can be arranged where at least a part of the first filter region and at least a part of the second filter region overlap each other. Namely, since each of the long-period gratings included in the optical filter according to the present invention secures the independence of its transmission characteristic, they can be arranged as being overlaid on each other within the same section along the longitudinal direction (aligning with the advancing direction of signal light advancing through the core region) of the optical waveguide, whereby the optical filter itself can be made smaller.
The method of making an optical filter according to the present invention having a configuration such as that mentioned above comprises the steps of preparing an optical waveguide having a core region doped with an impurity such as GeO2 or the like for changing the refractive index and a cladding region, provided on an outer periphery of the core region, with a refractive index lower than that of the core region; and irradiating a predetermined part in the core region of thus prepared optical waveguide with grating forming light (which is intensity-modulated along a generating a periodic refractive index fluctuation along the longitudinal direction of the optical waveguide). Here, the optical filter according to the present invention may be configured such that a plurality of optical filters each provided with a long-period grating having a desirable attenuation wavelength are connected in series along the advancing direction of signal light. Specifically, in the method of making an optical filter according to the present invention, a refractive index fluctuation with a first period is generated in a first grating forming region at a predetermined position in the core region, so as to form a first long-period grating attaining an attenuation peak at a first wavelength by mode coupling. Further, a refractive index fluctuation with a second period is generated in a second grating forming region at a predetermined position in the core region, so as to form a second long-period grating attaining an attenuation peak at a second wavelength, different from the first wavelength, by mode coupling.
In particular, in the method of making an optical filter according to the present invention, the first and second periods are set such that the order of cladding mode in the light attenuated by the first long-period grating to couple with the core mode thereof differs from the order of cladding mode in the light attenuated by the second long-period grating to couple with the core mode thereof.
Thus, in the method of making an optical filter according to the present invention, the first and second long-period gratings whose orders of cladding modes coupling with their corresponding core modes differ from each other are formed in a single optical waveguide or in a plurality of optical waveguides prepared for the respective long-period gratings. As mentioned above, the transmission characteristics of these long-period gratings secure their independence from each other, and an optical filter having a desirable transmission characteristic as a whole can be obtained.
In the method of making an optical filter according to the present invention, for reducing the size of the optical filter, it is preferred that the second grating forming region be set at a position where at least a part thereof overlaps at least a part of the first grating forming region.
Further, the method of making an optical filter according to the present invention preferably comprises the steps of measuring a transmission characteristic of the first long-period grating after the first long-period grating is formed; measuring a transmission characteristic of the first and second long-period gratings after the second long-period grating is formed subsequent to the forming of the first long-period grating; and computing differential data between first data concerning thus measured transmission characteristic of the first long-period grating and second data concerning thus measured transmission characteristic of the first and second long-period gratings as third data concerning a transmission characteristic of the second long-period grating. In this configuration, since the transmission characteristic of the optical filter is measured at each long-period grating making step, it can be confirmed whether the respective transmission characteristics of the first and second long-period gratings coincide with their designed values or not.
On the other hand, it has been known that, when an optical waveguide is irradiated with ultraviolet light having a predetermined intensity, the ratio by which the refractive index of the core region changes tends to be higher at the initial stage of irradiation and then converge onto substantially a constant level. Therefore, when the second and third long-period gratings are directly formed in the same grating forming region in an overlying manner, the amount of change in refractive index of the gratings would vary, whereby the overall transmission characteristic of the overlaid assembly may vary.
Hence, in the method of making an optical filter according to the present invention, the core region (grating forming region) of the optical waveguide is uniformly irradiated with a predetermined amount of ultraviolet light beforehand, and then is irradiated in an overlying manner with ultraviolet light which is intensity-modulated along a longitudinal direction of the optical waveguide. As a consequence, the ratio of refractive index change in the long-period grating formed earlier and the ratio of refractive index change in the long-period grating formed later become substantially identical to each other, thus making it easier to obtain an optical filter having a desirable transmission characteristic.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.